Effects of Iron Compounds on Batch Anaerobic Digestion of Primary Sludge: Control of Hydrogen Sulfide and Phosphate

This study evaluated the effectiveness of using iron (Fe) powder and ferrous chloride (${\mathrm{FeCl}}_2$) to decrease hydrogen sulfide (${\mathrm{H}}_2\mathrm{S}$) concentration ([${\mathrm{H}}_2\mathrm{S}$]) from biogas production by anaerobic digestion (AD) of primary sludge, and to decrease phosphate (${\mathrm{PO}}_4\text{-}\mathrm{P}$) concentration ([${\mathrm{PO}}_4\text{-}\mathrm{P}$]) in the sludge. [${\mathrm{H}}_2\mathrm{S}$] and [${\mathrm{PO}}_4\text{-}\mathrm{P}$] removal increased as doses of Fe powder and ${\mathrm{FeCl}}_2$ increased to a certain point. Optimal supplementation was $5\mathrm{g}/\mathrm{L}$ for Fe powder (${\mathrm{H}}_2\mathrm{S}\text{reduction}=98\%$, ${\mathrm{PO}}_4\text{-}\mathrm{P}\text{removal}=95\%$) and $0.5\mathrm{g}/\mathrm{L}$ for ${\mathrm{FeCl}}_2$ (${\mathrm{H}}_2\mathrm{S}\text{reduction}=98\%$, ${\mathrm{PO}}_4\text{-}\mathrm{P}\text{removal}=97\%$). Appropriate dosage of Fe powder or ${\mathrm{FeCl}}_2$ can control [${\mathrm{H}}_2\mathrm{S}$] in biogas and decrease [${\mathrm{PO}}_4\text{-}\mathrm{P}$] in AD effluent, but excessive dosage can inhibit the digestion. The biogas production and methane content were higher with Fe powder compared with ${\mathrm{FeCl}}_2$, but neither treatment had a significant effect on methane yield. The measured biogas production was accurately captured by the first-order kinetic model, as evidenced by a strong fit with high $R^2$ values (0.957–0.980), and the deviations between the measured and predicted biogas production using the first-order kinetic model were $<10\%$.